First off, in normal D&D wounds don't damage ability scores, nor do they inflict penalties to them, except if the attack has a rider effect of some sort that does so (like a bite attack that also delivers poison, or a weapon with the Exhausting enchantment).
Secondly, you should always differentiate between ability damage and penalties. For example, when you're exhausted, your Str and Dex are penalized by -6. You cannot get more exhausted than you already are, so the penalty cannot increase beyond -6 through exhausting effects. As soon as your exhausted condition expires (through rest, a Ray of Resurgence spell, etc.), that -6 penalty goes away.
If you're poisoned, though, you might take 6 points of Str or Dex damage. This damage stacks with itself (so you can be poisoned multiple times for cumulative effect), unlike exhaustion penalties. Also, ability damage does not go away when the condition that caused it expires. Casting Neutralize Poison will not restore any ability damage already dealt by a poison, for example. Instead, you need several days of rest (healing ability damage at a rate of 1 point/day), or magical healing, like the Restoration line of spells.
To pick an example similar to what you posted, let's say you have Str 16, and you're hit with a Ray of Enfeeblement that happens to inflict 6 points of Str penalty. You're then hit with a Ray of Exhaustion and fail your save, so you gain the exhausted condition which brings with it a further -6 penalty. These penalties, being untyped and from different sources, stack to produce a total penalty of -12. For all practical intents and purposes, your Str is considered to be 4 now, which sucks, until the Ray of Enfeeblement's duration expires.
Finally, you're getting poisoned with Wyvern venom and, this being an obviously
day, fail both your saves. Your DM rolls damage, and you take 2 points of Str damage immediately, and a further 7 points 1 minute later. These two instance of Str damage stack, because damage always stacks, even if the source (one dose of Wyvern venom) is the same. So you take a total of 9 points of Str damage.
Now what's your effective Str score? Actually, this is difficult to answer, because Ray of Enfeeblement has this little clause: 'The subject’s Strength score cannot drop below 1.'
I would read this as follows: Your damaged Str score is 7 (16 minus 9 points of damage), to which a total of twelve points of damage is applied. However, only 6 of these 12 points of penalty could theoretically lower your Str score below 1, while the other 6 points of penalty have a clause which forbids this. So your exhausted condition would be applied first (reducing your effective Str to 1), and the effects of the Ray of Enfeeblement are applied last - which means that, in this case, they're not applied at all, since your Str score is already effectively 1.
Other DMs may read this differently and apply effects impacting your ability scores in a different order (Ray of Enfeeblement non-last being the important part here), which would result in your effective Str score dropping to zero. It cannot drop below zero, however, because the SRD explicitly forbids this for all cases: 'Keeping track of negative ability score points is never necessary. A character’s ability score can’t drop below 0.'
Hope that was clear!
