Adjectival form of "collide"—"collideable" or "collidable"?

Short answer: There's no clear choice; take your pick.

Long answer: Neither collideable nor collidable is a word you're likely to find in a dictionary, but in your context using it (one of them) may be exactly the right choice. As for the spelling preference, Wikipedia's detailed article on American and British English spelling differences says:

Before -able, British English prefers likeable, liveable, rateable, saleable, sizeable, unshakeable, where American practice prefers to drop the -e; [borderline: tradeable, smokeable, driveable, shareable] but both British and American English prefer breathable, curable, datable, lovable, movable, notable, provable, quotable, scalable, solvable, usable, and those where the root is polysyllabic, like believable or decidable. Both forms of the language retain the silent e when it is necessary to preserve a soft c, ch, or g, such as in traceable, cacheable, changeable; both usually retain the "e" after -dge, as in knowledgeable, unbridgeable, and unabridgeable. ("These rights are unabridgeable.")

The "polysyllabic" rule would point towards collidable, but elsewhere, a search brings up the following poly-syllabic words ending in eable (other than soft c, ch, g instances, of which there are many) (I haven't checked their provenance):

canoeable diagnoseable disagreeable dislikeable fireable foreseeable handleable hireable machineable microwaveable removeable settleable throttleable unforeseeable unnameable upgradeable whistleable

Or you may want to look specifically at -able words formed from verbs ending in -de, and decide whether -dable or -deable is preferable:

abradable codable decidable degradable dividable evadable excludable extrudable fadable gradable guidable hidable includable persuadable ridable/rideable slidable tradable/tradeable upgradable/upgradeable wadable/wadeable

It seems that analogy with dividable and decidable (the closest?) would suggest collidable. (But if you still prefer collideable, it's fine to use it…)