Well, they're pack weapons so you can't react with them at all - but you can use the MI special reaction to take a ready action in the rare occasion that a Bug player gets within 10" of you with his second or third action and doesn't make PBR with the troops.
Then in your first action the next turn, you open up with bugshot grenades. Half as many dice perhaps, but you can get hits on the more substantial bug species and a fair chance of a few kills.
As for the mixed action business - surely if part of your squad is shooting flamers, the other lot have to take a shoot action also? I didn't know you could split actions amongst a unit, the whole unit must take the same action?
This occured to me when I had an Exo squad with one wounded man... he couldn't ready/jump, but could the squad shoot, then ready preparing to jump next turn? I know a wounded model can only take one action, but does it /have/ to be the first action, or can it optionally be the second (thus the whole squad isn't effectively screwed if they choose not to leave the wounded man behind)