This is a "why not" situation. If something can be improved upon, you improve it. For your difficulty concern, if you can defeat (or suicidally dive bomb the crater despite the) Meteor Heads, you can beat (or suicidally dive bomb past) the new enemies, too.
Really.
1) If it doesn't spread, the only alternate method is to make Meteorites even more destructive on impact.
2) In a game where magic (and soon aliens) exist, logical explanations for such phenomenon are futile and laughable.
"if something can be improved upon, you improve it."
ok, remove the worms. remove the spiked slime. change the slime's name to something like "irradiated rock" so it seems like a part of the meteor is attacking you. don't reuse other mobs.
"if you can defeat meteor heads, you can defeat the new enemies. "
no, worms can get to you through blocks, and you can't exactly dodge them. it's pretty hard to. and slimes jumping on top of you and shooting you is pretty crazy too. it's hard enough to mine when meteor heads are constantly flying into you. adding more enemies would make meteorite mining a nightmare.
"if it doesn't spread, the only alternate method is to make meteorites even more destructive on impact."
ok, good point, but only to a point. 50 blocks is kinda much, especially for small worlds. maybe change it to 30 blocks or so.
"in a game where magic (and soon aliens) exist, logical explanations for such phenomena are futile and laughable."
this reminds me of the "drink water while drowning" thing. no, it's not perfecly logical, but it does try to make sense. you find trees on the surface, notsideways in the underworld. you craft an anvil from iron, not sticks. it needs to make some kind of sense, and meteorites spreading doesn't. maybe it automaically converts it when it spawns, through radiaton or something.