Help with 3.5 monster summons lists

[Rainshine]

Hey there! I was hoping perhaps some of you could lend some tips: I'm trying to make up some custom monster summon spells for D&D 3.5. Some of the spells would be specific summons, for example: Summon Stone Giant. The other situation is giving spontaneous themed summons to a prestige class -- by burning a level 4 spell, they can cast summon monster IV to get a fire creature.
My problem is, I'm not real sure what CRs a given spell level should give access to. I've seen Summon Monster V lists with creatures from CR 3-6. And there's not a lot of Water creatures down at the Summon Monster I and II level -- stuff below CR 2. Small Water Elemental is CR 1, but other than that, I'm on my own to make up some interesting creatures, it seems. I don't mind it that much, but when I'm doing this for a player and then for several NPCs, that's even more custom monsters to keep track of...
Suggestions?

[Debihuman]

Rather than giving yourself a headache, simply add more monsters the the existing spells. Compare CR ratings (there is a range) and just slot each new monster in the the appropriate Summon Monster spell. It's a lot easier than forcing players to waste a spell slot.

I believe Unearthed Arcana gives alternate spell lists for underwater creatures but I'd have to check it to be sure. It would be under Summon Monster variants page 136. Stormwrack might also have some suggestions.

Debby

[Fizban]

The basic Summon Monster/Summon Nature's Ally lists have no set rules. They're balanced based on the individual creatures, not by CR, hit dice, ECL, or anything else, and you'll pretty much just have to eyeball and playtest it. There are lots and lots of suggested substitutions in various monster books, so if you check out one of the Summoner's handbooks for a list of what creatures you can summon with what spell, you should be able to get a good idea of what's okay at what levels. I'm going to have to disagree with Debihuman here: you do not want to just add more to the lists. More versatility always amounts to more power in the hands of a clever player. There's actually a sidebar about this in the Planar Handbook right before they tell you what creatures can be substituted for what without skewing the lists.

As for summoning a single creature, those have a very clear pattern. A spell that summons a specific creature (like your Summon Stone Giant idea), requires concentration, and lasts a maximum of 1 round/level, usually summons a creature with CR equal to or one less than the minimum level required to cast the spell. So a 6th level spell would summon a CR 10 or 11 creature for as long as you concentrate. You'll still have to use your own judgement, as this isn't what the CR system is meant for, but if you look at the spells that do this (Summon Hound Archon, Summon Greater Elemental, Summon Aspect of Bahamut, and so on) then you should be able to get an idea of what should be okay.

I'd make a prestige class summoning ability a bit stronger than normal, relative to whatever kind of summoning you're basing it on. An ability that works like a Summon Monster spell would summon from a unique list, and if it was summoning only one type of creature then I'd let it bring things that are one "list level" more powerful than if you had a choice in the matter. A summoning based on concentration might last for a few round after you stop concentrating, or summon a creature with a powerful 1/day ability so that it's almost like 2 spells in one-the ability and the leftover creature.