Designing the Gnobbit, the Gnalfling, or the Halfome.
(Note: Neither Svirfbits nor Halfieblein are addressed. That is left as an exercise for the reader).
For a "balanced" hybrid, this fits within the general "balanced" for AD&D 1e that exists.
Demi-Human breeds with Demi-Human
The two 1e races are both in the small size, and are very similar per the PHB / Unearthed Arcana. The proposed plusses and minuses are both similar to and different from the Half-Elf and Half-Orc limits and bonuses used as a point of reference.
Character Class limitations (Table I)
Per Gnome and Hobbit as printed with two adjustments:
- Yes to Assassin (not usually a Halfling class)
- Yes to Druid (not usually a Gnome class)
This reflects a "best of" fusion of half-breed mixes. Class prohibitions aren't a balance issue with a level here and there.
Class Level Limitations (Table II)
The Strength stat limitation below informs limits on a Gnobbit Fighter, and both parent races have unlimited Thief potential. This suggests Halfhome Assassin potential to match that of the Gnome. Since Halfling Druids would exceed by one the level for a Gnome Cleric, based on Wisdom, there's no point in restricting either a Gnalfling Druid or Cleric.
Ability Score Min/Max, Character Race Table III
Minimum ability and maximum ability scores:
- Str limits per Halfling
- Wisdom limits per Gnome
This balances out as a net 0, just as the Half Orc +2 / -2 balances out as a 0.
No adjustment. It's a beautiful baby. The -1 for Gnomes is overcome by inherent Halfling cuteness. (Full disclosure: I played a lot of Halfling thieves in 1e ...)
Beyond that, Comeliness isn't a balance issue at one point since Comeliness scores go well beyond where other stats go via Charisma based modifiers.
-1 Str / +1 Dex, per Halfling. This is a net zero. Since the gnome has no racial stat increases or decreases, this fits the KISS principle.
Saving throw bonus
a. These two races are nearly identical in the Constitution based saving throw bonus method for spells, so retain it as is with the same ratios based on Constitution.
b. The Special Poison Save: take the Halfling. (A few other things will be debuffed to compensate for this).
This recommendation is made due to how Constitution is a logical common race strength and logical in poison resistance at the concept level of what Constitution represents in 1e. (See DMG discussion of what abilities simulate).
Use the 30' infravision, per the fraction of Stouts who have Dwarfish blood getting 30' infravision. This is part of the ""reduce to offset the Halfling poison save bonus," versus Gnomish 60'. There's a price for being so good looking! :-)
Per Gnomes, except subtract 3 not 4 from giant based attacks. (Halflings get no bonus). This is another minor adjustment to account for that poison save.
In melee combat, gnome characters add 1 to their dice rolls to hit opponents who are kobolds or goblins. When being attacked by gnolls, bugbears, ogres, trolls, ogre magi, giants, and/or titans, gnome characters subtract 4 from their opponents' "to hit" dice rolls because of the gnomes' small size and their combat skill against these much bigger creatures.
Underground terrain, slopes, and direction finding
Per Halflings who are Stouts with Dwarf blood. (That's not as good as Gnomes).
Per Halfling, except it surprises on a 1-3 versus 1-4 on 1d6. Normal surprise is on a 1-2. Still a nice bonus, but reduced as the final compensation.
Is such a hybrid supported by 1e as a system?
It is internally consistent from a mechanical sense although it is neither explicitly addressed nor "allowed."
Two different races orcs (humanoids) and humans, as well as elves(demi-humans) and humans breed to make half-orcs and half-elves. Demi-humans seem to be in the same genus/species, or whatever taxonomy drives this. This PoV is based on categories of human, semi-human, demi-human, and humanoid. (DMG page p. 16 for demi-humans, DMG p. 106, HUMANOID RACIAL PREFERENCES TABLE).
Note: from PHB, p. 17, only 1 in 10 half-orcs born are sufficiently non-orcish to fit the Half-Orc racial category of the PHB. There is reference to some Halflings being "more Stoutish" as a similar "how mixed is my blood?" reference.
From all of the above, a ruling of "Yes" makes sense.
There are explicitly stated limitations on other demi-humans, semi-humans, and humanoids in 1e, see the PHB tables referred to above. DMG p.18 places further class limits for some races.
The larger argument against is the explicit statement that AD&D is played in a humanocentric world. From that you could argue that the only half breeds that fit in the general theme are human/xxx half breeds, since those are the only two explicit examples given.
From this PoV the "No" ruling fits the authors' intent, since no explicit provision for this cross breeding is made, unlike the half-orc and half-elf cases.
It's a human's world. Based on Gygax' various writings, my take is that he was trying to address what JRR Tolkien discussed in "On Fairy Stories" regarding the overlap of the primary(mundane) and secondary(magical) worlds. He did this to make running the game easier. (As complicated as AD&D can get, there was some attempt to mitigate the problem).
From DMG page 21 (excerpts from a larger discussion)
ADVANCED D&D is unquestionably "humanocentric", with demi-humans, semi-humans, and humanoids in various orbits around the sun of humanity. Men are the worst monsters, particularly high level characters such as clerics, fighters, and magic-users - whether singly, in small groups, or in large companies.
While there might well be some near or part humans with the group so doing, it is certain that the leaders will be human. In co-operation men bring ruin upon monsterdom, for they have no upper limits as to level or acquired power from spells or items.
The game features humankind for a reason. It is the most logical basis in an illogical game. From a design aspect it provides the sound groundwork. From a standpoint of creating the campaign milieu it provides the most readily usable assumptions.
From a participation approach it is the only method, for a11 players are, after all is said and done, human, and it allows them the role with which most are most desirous and capable of identifying with.
Rule from whichever perspective makes the most sense to you.