Could I summon, say, 8 Sprites and have them surround me, occupying the eight spaces around mine, while I whale on goblins with my glaive or other reach weapon with near-impunity, since the goblins would be unable to approach? I'd still have to deal with their ranged attacks, but if I'm also invisible, I should be nearly invincible vs. Small enemies (at least until they kill off my Sprites).
First of all, you are totally correct that RAW, a small creature cannot move through a hostile tiny one's space. As I mentioned elsewhere, some DMs may decide that this rule doesn't apply to all tiny creatures, but RAW this is accurate.
There is a slight complication: tiny creatures only control a 2.5 square foot of space in combat (PHB, p. 191), and you need to keep the goblin 5' or more away from your space.
Still, if you are using the rules for movement on a grid, a DM would probably consider the entire square to be occupied by the sprite's space (as there are no clear rules on how decide the sprite's location within the square). And if not, you could place the sprites like so (orange is sprite's space, green is you).
Why this tactic is even better than you think
You wouldn't just be safe from small enemies using this tactic, but larger ones as well (as long as they have a range of 5' or less). Note that the rules on stopping your movement in another creature's space means it is against the rules for a creature to move into a creature's space, attack (anything), and move away regardless of the size of either creature. So your ring of sprites would protect you from melee attacks from larger creatures as well.
Why this tactic isn't so good
Invisibility doesn't make you immune to ranged attacks. If enemies know where you are (which they will unless you Hide, and even then they might be able to guess if you're surrounded by a ring of sprites) they can target you with disadvantage.
Most tiny creatures (like sprites or fairies) have only 1 or 2 hit points, and not great AC (e.g. 15). At best, you'd be warding off a couple of attacks with this tactic before enemies can swarm you again. Fewer if any of your foes can deal area-of-effect damage (e.g. cast shatter).
You're using up two 4th level spell slots for this tactic (conjure woodland beings and greater invisibility), both of which require concentration and thus can't be cast by the same person, and both of which require an Action to cast. Many spells can get you out of melee range (flight, spider climb, blink) or effectively cut off access to you (wall of fire, spirit guardians) for much fewer resources than this. Given the defensive boost you'll likely get getting (only warding off one or two attacks from the goblins), you'd probably be better off doing nothing for your defense, but casting shield if an attack gets through. The +5 to your AC will almost be as good a defensive boost as this tactic, and for far fewer resources, and it won't restrict your actions at all during your turn.
Sure, it’s no different from standing behind party members.
The only real issues are maintaining invisibility while ”whaling” and that the goblins only need to dispose of one sprite to get at you. They have lots of ways of doing this: killing it, shoving it, grappling it and pulling it away etc.
If you had 8 sprites, yes. Until the goblins hack through them. But the summoning spells your thinking of, as written, lets the DM choose which specific creatures are summoned. From the Sage Advice Compendium:
When you cast a spell like conjure woodland beings, does the spellcaster or the DM choose the creatures that are conjured?
A number of spells in the game let you summon creatures. Conjure animals, conjure celestial, conjure minor elementals, and conjure woodland beings are just a few examples.
Some spells of this sort specify that the spellcaster chooses the creature conjured. For example, find familiar gives the caster a list of animals to choose from. Other spells of this sort let the spellcaster choose from among several broad options. For example, conjure minor elementals offers four options. Here are the first two:
- One elemental of challenge rating 2 or lower
- Two elementals of challenge rating 1 or lower
The design intent for options like these is that the spellcaster chooses one of them, and then the DM decides what creatures appear that fit the chosen option. For example, if you pick the second option, the DM chooses the two elementals that have a challenge rating of 1 or lower.
A spellcaster can certainly express a preference for what creatures shows up, but it’s up to the DM to determine if they do. The DM will often choose creatures that are appropriate for the campaign and that will be fun to introduce in a scene.
Which might get you Small or even Medium creatures without the ability to turn invisible. Also, ranged attacks (goblins with short bows) would be able to slip by tiny creatures.
Is it worth it?
You're proposing a plan that costs 2 actions and two fourth level (greater invisibility and conjure woodland beings) spell for use on something as low-level as goblins. Blink would be a better option to attain almost the same thing (with the risk of not bamfing out on a given turn).