I cannot get a power source to sit in the MK3 equipment grid. Is there a certain requirement?

There is no difference in requirements from the base game equipment.

Could it possibly be some other equipment mod that changes base game equipment that isn't expecting bob's mods?

It does not allow me to insert into MK4 armour items from Krastorio 2: power source (both nuclear and "normal"), roboports, batteries. I wonder is this happening because these items miss some property that power armor expects? I hope it just does not list compatible items...
On the other hand - I was able to insert Exoskeleton (lev. 2) and energy shield without any issues, and they seem to also be altered by Krast mod. But not replaced as the other stuff... Hmmm... I guess it's more issue with Krastorio then?

It does not allow me to insert into MK4 armour items from Krastorio 2: power source (both nuclear and "normal"), roboports, batteries. I wonder is this happening because these items miss some property that power armor expects? I hope it just does not list compatible items...
On the other hand - I was able to insert Exoskeleton (lev. 2) and energy shield without any issues, and they seem to also be altered by Krast mod. But not replaced as the other stuff... Hmmm... I guess it's more issue with Krastorio then?

I'll take a look and let you know.