But on the other hand, this power does leave some stuff out. For example, grappling a swarm is pretty easy if this is all they've got to represent swarm-ness.
I feel like that should be a separate power, to be honest, to allow greater flexibility as to what this power represents. A vine-demon could have Swarm Body on the basis that each feeler destroyed is just an insignificant fraction of its form, and you need to hit
all of them to do serious damage; but it wouldn't be automatically better at creeping under doorways.
I'd leave this power as it is, and then apply another power with the base effects (not the upgrades) of your Amorphous, which represents that effect rather well (Amorphous was, in fact, going to be the name for this one, before I decided that Swarm Body fit better).
Also, I consider Physical Immunity to be drastically undercosted. Balancing against it is probably unwise.
I originally costed this on gut-feeling, then compared it to Supernatural Toughness (on the one hand, outperforms in terms of raw damage reduction at hits of 4+ stress. on the other hand, does not increase stress track. on the third hand, has a really simple Catch), and
then to Physical Immunity. I think it's alright where it is, to be honest. Each time I consider raising it to [-4] I wince over the Catch.
PS: There's a thread on the Resources Board for custom powers.
Oh? I thought that was for archived stuff?
EDIT:
I'd basically take my Swarm Body and add it to this version of your Amorphous power:
AMORPHOUS [–1] Description: Your body is rubbery, or semi-liquid, or composed of a swarm of insects; whatever the case, barriers offer little resistance.
Skills Affected: Might, Athletics, other physical abilities.
Effects:No Hole Too Small. Fences are hardly an obstacle to one who can slip through the posts. You can ignore all physical barriers that are not completely sealed. Magical barriers, such as thresholds, still impede you as normal.
Look Ma, No Handholds! It’s difficult enough to grab a normal opponent, let alone one who slips and wriggles through your grip. The effective Might (or other relevant skill) of any character attempting to grapple you (or maintain a grapple) is reduced by two for that purpose.