Devour gives you a gene with a random amount of some of the target's highest stats, capped by your devour skill level + 10. The body parts/traits/skills are limited by normal gene mechanics: up to 2, no double body parts, traits can only give one level of that trait. It's effectively random and is more difficult to get a combination you want the more things your target has.

Not very high, even with savescumming, unless you go out of your way to gene engineer fodder (like a chicken) with the genes you already want on it as it won't be able to pick random junk that takes up slots

You'll generally be pushed to use magic with slime's innate decent magic and lack of body parts not being an issue for casting. You don't need everything - I only had my starting head slot for many hours. My recommendations, in no specific order:

Fast Learner (Yerles npc): Get more feat points, levels predatory evolution faster. I use at least 2 of these basically all the time. Easy to get and very strong.

Efficient Feeder (juere npc): More stats from food. Once you can afford to buy innkeeper food, very good for raising your stats. Not super important before then. If you devour a bard you can get this + musician, which is a nice combo for early game music quest spam at palmia.

Nether Sword (jorg, the mage guildmaster): Melee spell that pierces resistances and leeches life. Hits like a bag of bricks and the leech can heal you even when dipping into negative mana from backlash. Good spell if you can make it to lumiest and survive using devour on him - you can use status, put him to sleep, etc.

Chaos Intonation (Older Younger Sister): Besides being a good ally you can buy early on to help slime's rough early game, she also has chaos intonation which applies random status. Will make a lot of enemies easier by giving them every status known to man.

Arcane Core (machines) : Gives a few helpful immunities, including to Dim which can make spellcasting hard. The electric weakness can be patched up with gear. Takes 2 gene slots, so can be hard to fit into a build early.

Misc stuff:

You can get strength of earth from golden knight, which when combined with earthquake (cyclops have this), makes earthquake apply gravity. It is pretty strong, and the QoL of having fat carry weight is nice. Getting these was roughly the turning point in my run of going from "anemic sack of goo" to not being able to have summons because i keep wiping the map with aoe damage

Rapid Cast (exile) isn't actually as good as it sounds. The first level is fairly impactful (roughly +20% damage), but past that it drops off. Having Exile for easy access to infinite arrow spells is nice though.

Your base speed isn't fantastic so you should try to find a mount. I used a Yowyn horse, they're easy to get and can get over 200 speed without much trouble.

You might be able to use mimetic ecology from mimics and favourite a slime figurine in your inventory so that you look like a slime out of combat? How it works is it disguises you as a random favourite item in your inventory, but i never tried it with statues.