I had quite a bit of alchemy ingredients while in a dungeon containing an alchemy table. My problem is that for many ingredients, if you only have a few types, they'll only produce whatever potion they choose, instead of what I want. For example, when 2 ingredients have 2 reactions that are the same, the game decides which kind of potion will occur, If I click the Fortify Sneak, and both ingredients also have Lingering Damage Stamina, the game will pick the Lingering one instead. Is there a way to change this? It's not such an issue when I get to my massive collection in the house, but out in the field, that doesn't cut it.

The last perk in the Alchemy tree is Purity, which removes negative effects from potions and positive effects from poisons. To get it, you have to climb the Alchemy perk tree by either taking Alchemist > Physician > Benefactor > Experimenter > Snakeblood first, or Alchemist Physician > Poisoner > Concentrated Poison Snakeblood, depending which side you choose. And of course your Alchemy has to be at 100 to get the perk once you get the pre-requisites.

If you combine 2 (or more) ingredients with 2 effects in common, BOTH of those effects will be in the potion (read the description, not the potion's name).
In your example, combining 2 ingredients with BOTH having fortify sneak + Lingering damage stamina, will create a potion that will have both of these effects, regardless of how the potion is named.

Here's an example: Blue Mountain Flower + Wheat will give you a Fortify health + Restore Health potion, even though this potion might be named just "Fortify Health Potion"

Look http://www.uesp.net/wiki/Skyrim:Useful_Potions for a list of useful potions. UESP.
Or http://skyrim.melian.cc/?cmd=cmdSkyrimAlchemyWizard for an Alchemy wizard. You choose what effects you like and it gives you the available ingredients for it.

Thanks, lorelai. Of course, that would make the potion practically useless, so I guess either way, I'd have to wait until I had 2 ingredients compatible with a wanted effect. Thanks so much for the links. I raised up Alchemy to 100, with almost all the Perks, as I found it immensely helpful in Oblivion, and I guessed right, now that I have 103% sneak for 129 seconds, or 103% Marksman bow damage for 60sec. plenty enough to drop an Elder Dragon in 3 or 4 shots. hehe. ( And I need all the help I can get!)

Not quite entirely useless. Those extra unwanted effects add to the resale value of a potion. Which can be quite useful.

DON'T TAKE THE LAST ALCHEMY PERK!
It'll make the potions worth a lot less.

How did you concoct 103% bow damage for 60 sec?

Peerless Alchemy on a circlet and a necklace, and Extreme Alchemy on glass gauntlets. I haven't gone and made any super-powered alchemy armor yet, since I doubt I'll need more potent potions than I can make now.

I did take that one, but I've got enough gold to choke a horse. Anyway, Damage Magica Regen. is worth over $1300 gold, and I made a lot of them for sales. I scoop up every bit of ingredients from Alchemy sellers, and then take the rest in potions I'll sell to make the difference, or in trade for magical rings. necklaces, etc. I've slowed that stuff down a bit, 'cause I've done most of the side quests, and just became Arch-mage, but I'm less than halfway through the main quest.

^^ Glad I could help!

I never take the Purity perk; but not because it lowers the price of potions - heck you get Purity at 100 - so you no longer need high priced potions to level, and you are pretty much covered in gold by that time.
The reason I don't take it is because I find it a waste of 1 perk point that can go somewhere else. With all the ingredients in the world I really don't need that perk to brew whatever I desire.