LA doesn’t come into play; RHD is murkier
ECL matters only for encounter balancing, WBL, and XP. DMs are instructed to consider you based on your ECL when determining appropriate CR for encounters, when determining how much XP you get from the encounter, and how much XP you need to level-up again. When considering your “wealth by level,” ECL is the level in question. Outside of these purposes, ECL is never used.
Level Adjustment and Effective Character Level
Use ECL instead of character level to determine how many experience points a monster character needs to reach its next level. Also use ECL to determine starting wealth for a monster character.
(Monsters as Races)
Since ECL is the only figure that includes LA, this means that LA doesn’t count when trying to take 1st-level-only feats.
Contrast this with what the rules say governs feats:
Feat Acquisition and Ability Score Increases
A monster’s total Hit Dice, not its ECL, govern its acquisition of feats and ability score increases.
This doesn’t explicitly get into 1st-level-only feats—it couldn’t, since they didn’t exist at the time this was printed—but even at maximally pedantic, the “acquisition of feats” could reasonably cover requirements as well as when you actually are allowed to select feats. And 1st-level-only requirements certainly aren’t covered by the few things we’re instructed to use ECL for.
Level Adjustment—no problem
That means that LA does not count against your “level” insofar as 1st-level-only feats are concerned. So a 1st-level aasimar monk could indeed take Troll-Blooded, even though they are ECL 2nd at the time they take their 1st class level, gain their 1st HD, and select their 1st feat.
Single RHD—no problem
Creatures with 1 or less HD replace their monster levels with their character levels.
Therefore, any creature with a single listed HD in its statblock is no problem—when they are a 1st-level monk, they also have only their 1st HD, and so have no trouble taking 1st-level-only feats.
By the same token, racial hit dice—which are included in one’s “total hit dice”—do count for feat acquisition, which means a lizardfolk monk is “3rd level” for this purpose when it actually takes the monk level and gains Toughness. Creatures with more than 1 RHD don’t convert those RHD to class levels, and
If a monster has 2 or more Hit Dice, it can start with no class levels (though it can gain them later).
This is a concern here, because that implies that our 2-HD lizardfolk exists like that—with its 1st-level feat but without its monk level—and so the 1st-level feat has to be chosen without Toughness, and so Troll-Blooded cannot be selected.
However, none of the rules actually describe this. The rules for starting at a higher level, or for playing a monster, are rather vague. Indeed, the above quote only states that such a PC “can” start with no class levels—it is much less clear that they must.
The usual process of creating a character follows a specific order: ability scores, race, class, skills, feat, also known as Chapters 1, 2, 3, 4, and 5 of the Player’s Handbook. The rules for creating a higher-level character don’t change this: they indicate that step 2 in the process is “Determine race and class,” (Dungeon Master’s Guide, pg. 199). That not only includes one’s first level, but actually all of the levels one is taking when starting; multiclassing is explicitly addressed.
Step 3 (ibid.) says “Determine character statistics. This includes […] feats.” No more information is given. Nothing explains how or when you choose your feats, and what you can and cannot consider for your prerequisites. Any sense that your first feat has to be one you could have gotten based on what you had at 1st level isn’t mentioned. There is a certain presumption that characters that start at a higher level are supposed to be the same as characters that started at 1st and leveled up to the given level, but that’s not spelled out.
And the monsters-as-races rules don’t address this at all. The only time the word “feat” is used at all in the description is that the example ogre with no class levels has “two feats,” with no description of how those may or may not be chosen.
The glaring lack of rules doesn’t really give us a whole lot to stand on. There are effectively two options:
We assume everything that says you choose race and class before choosing feats—e.g. from the usual character-creation process without RHD involved, as well as from the description of starting at higher level—still applies. Thus, the character is, for example, both a lizardfolk and a monk before any feats are selected.
We assume that a higher-level character is supposed to match a character created at the lowest possible level, and that has leveled-up from there, and furthermore that for a powerful race this means that they started without class levels, which is something they “can” do. In short, we assume that no only “can” they, but in some sense they must do so, or at least be worked out as if they had done so.
If we go with option 1, that means you choose your race—lizardfolk—and then you choose your class—monk—and gain Toughness. Only after that are you presented with the opportunity to select feats—and you select both your 1st-level and 3rd-level feats at that time. That means that, yes, you have Toughness, and can select Troll-Blooded as your 1st-level feat.
If we go with option 2, you start as a lizardfolk with 2 RHD and no class levels. You select your 1st-level feat—from that first RHD—at this point, and don’t have any levels in monk (or any other class), so you cannot meet the Toughness prerequisite for Troll-Blooded and cannot take that feat. You then choose your first class level, gain Toughness, as well as your 3rd HD, so you gain another feat—but you aren’t 1st-level anymore, and so still cannot take Troll-Blooded.
Option 2 is what most people seem to assume—your higher-level character is supposed to match a character leveled-up during play. However, nothing says that, nor does it say that you must consider your lizardfolk as if they started as a class-less lizardfolk who later leveled up. It only says you “can” do that. Certainly, aside from RHD, all the rest of the rules would usually agree that you should have both race and class features available when meeting the prerequisites for your 1st-level feat. I wouldn’t allow a character starting at 12th-level to choose all five of their feats as if they always had everything they have as a 12th-level character—although that is a possibility that the rules for starting as a higher-level character seem to allow—but for the situation of feats on a character who has RHD and requires just that 1st class level to take feats they gained from RHD, I absolutely would allow it.