I'm wondering now how much Mines of Bloodstone influenced your DMs choices and rules. For example, in the H series, many of the fiend lords were boosted to 400hp when on their own plane to give them some chance of survival. Even backed up by say 12 pit fiends, I'm having a hard time imagining an unbuffed Geryon daunting a 20th+ level party, as he's hardly one of the more fearsome fiend lords. Then again, maybe by 'overwhelmed' you mean, "One of Geryon's Legions arrived.", in which case did combat sometimes get sloggy and how did you deal with it? Twelve beholders is after all a lot of ray attacks per round.
I don't recall exactly how much backup Geryon had, but 12 pit fiends would probably be in the ballpark. It was a pretty nasty fight. Assuming each pit fiend used its
hold person spell-like ability right away, that would mean each party member would have to make six saving throws vs. spell in one round to avoid being paralyzed. (3 targets per spell x 12 pit fiends / 6 party members) Even if you generously assume that all party members only fail a save on a natural 1 (which wasn't the case), odds are that two party members would be knocked out of the fight in round one.
IIRC, devils are immune to fire and cold and take half damage from lightning. Add in magic resistance and a highly likely saving throw and most of the raw damage spells are a lot weaker than usual against them. Often times, the best way to damage them is for a buffed up fighter to beat on them with weapons.
I remember our DM used to take the "may use once per round" spell-like abilities literally. As in, the monster could use one of those abilities per round and still do something else, like attack or cast a spell. I don't think that's the way it was intended to be played, but that's the way he ran it, and it made demons and devil more powerful by effectively doubling their actions per round.
The rule about magic items losing effectiveness through planar travel is found in several places in the Manual of the Planes. Page 81 has the rules for the outer planes. The biggest effect is that all +X items lose one "plus" per planar boundary crossed from its home plane. That means a +3 sword from the Prime Plane would only be a +1 sword in the Abyss (Prime -> Astral -> Abyss).
There are also rules in the Manual of the Planes for how planar travel affects class abilities. Clerics in particular get hit hard if they travel to outer planes far removed from their deity's home plane. They can only re-memorize 1st and 2nd level spells and their turn undead ability gets reduced. Conversely, evil clerics and creatures with turning ability have a pretty good shot of turning paladins in the lower planes.
In general, our party felt almost indestructible on the Prime Plane, but quite vulnerable in the Outer Planes.
On the topic of dual-classes, one of the best is Cleric 9/Magic-User X. A 9th level cleric only requires 225k xp, which is only six-tenths of a magic-user level past name level (375k). For that six-tenths of a level, the magic-user gets d8 hit dice for 9 of his 11 HD, excellent turn undead chances (second best column on the table), 4/4/3/2/1 cleric spellcasting (including healing magic, a magic-user blind spot), and the ability to use clerical scrolls and magic items. Also, his fighting ability and weapon choices are somewhat improved, which is nice. Saving throws also benefit a bit in the short run. The character can also qualify for clerical followers and a stronghold. Since clerics have a much better survival rate at low levels, I think it's one of the best ways to develop a magic-user while only slightly impacting his magic-user level in the long run.
Druid 12/Magic-User X is also worth looking at. It only costs 300k xp, which is still less than one magic-user level past name level. It grants access to every druid spell up to and including 7th level, and a total of 24 druid spells per day. The character also gets druid shape-shifting, 12d8 hit dice, and better combat abilities. The only real drawback is the long slog up to magic-user 13: 1,125,000 xp, which is a lot more than the 250k needed for magic-user 10 for the Cleric 9/Magic-User X option. Once the slog is done, though, you're less than one magic-user level behind a single-class magic-user.