With regard to HD, I think the intent of the spell is very clear that the HD of the summoned creatures, as well as the type (and in some cases number), are choices the caster makes. It is possible to gate in something you can’t control, but I don’t think the DM should have that happen against the caster’s will without letting them know well in advance that the is the new house rule.
In general, I don’t see the problem with characters using this power over others they might use instead. Sure it is powerful- it is also a 9th level spell. However, if it seems a problem, I do like the idea of gated in individuals not taking kindly to it- there is nothing in the spell that suggests they can’t. I would work that in slowly, though, rather than have 20 Balors turn up for revenge. That seems a little unfair. Maybe even warn the players that there might be consequences to persistently controlling powerful creatures. They might take the hint, cut back a bit, and problem solved.
In general, I don’t see the problem with characters using this power over others they might use instead. Sure it is powerful- it is also a 9th level spell. However, if it seems a problem, I do like the idea of gated in individuals not taking kindly to it- there is nothing in the spell that suggests they can’t. I would work that in slowly, though, rather than have 20 Balors turn up for revenge. That seems a little unfair. Maybe even warn the players that there might be consequences to persistently controlling powerful creatures. They might take the hint, cut back a bit, and problem solved.