He could summon anyone but the more powerful the ninja he summons the greater the cooldown before he can summon someone else.
For example if an S Rank ninja is summoned the cool Down until he can summon another would be a Month.
A genin and chunin about would have a cool down of week.
SSrank Ninjas or jinchuiriki would have a 6. Month cool down.
Honestly it also depends on what kind of interpretation you go for here when it goes to some techniques.
Like go with more extreme fanon explanations (or more positive canon ones pretty much) on Kawarimi, Henge and in a smaller way Bunshin, and you can do some really damn impressive things with them alone.
So reasonably well trained academy student may be actually pretty good 'parahuman' by Worm standards.
Also in canon Henge is in fact true shapeshifting, so... yeah, saying that this have potential for power abuse is an understatement in the hands of the right munchkin.
Kishimoto presented those as very basic skills, barely explained them in detail, and had really no idea what he introduced in his setting, and how strong those can be. Most fanon explanations actually nerf them pretty hard.
Heck, Bunshin (especially if you fix lack of shadow problem or use it outside of sunny places/light) mixed up with more normal clones can be pretty damn confusing to deal with.
Clones alone (pretty much all types) can be bonkers. That Earth Clone that runs at you? Whoops, it's not the real guy. Oh, what is that piece of paper and those strange Kanji letters on it? AH, PAIN, EXPLOSION!
Yeah, exactly. Also Itachi and his ability to add explosions to the clones he summons... proves that you may add chakra to clones, and make them explode like suicide bombers.
I duno kurenai would be much more terrifying than itachi. Because lets be honest, he was an unimaginative emogoth. Plus kurenai is much cheaper.
Also if fillers are available - you have Yakumo Kurama. You know, a girl with illusions so damn strong they literally affect reality, and work more like summonings, than illusions. Just summon her, give her something to paint with, and watch fireworks.
Also 2nd Mizukage. You know, guy strong enough, while being only GenJutsu user, to get a deadly draw with
Muu, guy with a stealth ability so bullshit I wouldn't be surprised if he received Stranger 10-12 by Worm standards, and Dust Release technique that disintegrates everything on subatomic level, and possibly one of the things I'm confident should work on Endbringers (although he may only have one shot at it, because if he misses Endbringer core, it's going to be bad), because in the end they are made ouf of very strange, but still somewhat organic matter.