Aphids much prefer lush green growth which often is an indication the plant is being fed too much Nitrogen. Aphids are easy to knock off the plant with a sharp stream of water, low pressure because the water from the nozzle of your garden hose can do severe damage to the leaves, more the Aphids.
Bone Meal will not be available to the plants for some time, depending on when the soil bacteria can convert that to usable plant nutrients. The same with the Alfalfa, which also will, eventually provide and excess amount of Nitrogen.
Like tomatoes peppers need warmth when growing up as well as an evenly moist but not too wet growing medium. What was different with the peppers versus the tomatoes?

Thanks - these peppers sure aren't lush green growth - most of them are only 2-3" tall. It seems to be mostly the Numex Sunrise and the Hot Portugals - the other peppers are OK and I only found 1 tomato leaf with aphids (that could have been b/c they were left in the back of the car last night in the garage, figured it would be warmer that way).

Used the same cheap (EarthGro? Only one I could find at Walmart) seed starting mix in March, the same MG Organic potting soil when transplanted into 4" pots. The same greenhouse. The tomatoes are 8-12" tall, the Hinkelhatz peppers are 3-4" tall, the Thais didn't germinate well at all but the 6 I have are 2-3" tall, no aphids, the Hot Portugals germinated pretty well but didn't survive transplanting/aphids so the 4 aphid-free ones I have are 1-2" tall and the 4 with aphids are barely 1" with curling leaves and I don't think they'll survive.

The sweet peppers didn't seem to be affected at all (though those aren't really tall either). Everything seems really wet - I think they were doused too much/often and that's why the low germination/repotting survival rates. I wasn't the one watering them - Uncle Harry who has run a commercial nursery for many years was taking care of them while they were in his greenhouse. I just went to seed and repot, then to pick up.

Though my cousin seems to have done well with his conventional MG plants started in Feb - or maybe he just started a lot more than I think and/or it was warmer then (heated). His stuff he started in March with me doesn't seem to have done that much better than mine - tomatoes may be a little taller, but peppers didn't do well, I don't know which/how many of his have aphids.

I just gave all the infested ones a good spray with water from a squirt bottle, they're in quarantine in the back of the garage (still get light b/c we have a half-glass door). Won't the aphids just fall to the soil and climb back up? I'm assuming some will survive the blast. How soon can I take the plants out of quarantine?

So, what's a good quick fertilizer (since I'm assuming that these plants aren't suffering from an oversupply of N)? I've got to get some growth on these peppers before putting them out, I moved the smallest ones inside the house, rest are in the garage, I will put them all outside whenever we get sun, leave them in the garage with doors open for a good part of the day, and take the smallest back in at night for at least the next 3 nights, maybe longer, so they're at 68+ during the day and 65 at night instead of 55-60.

Thanks for your help.

For aphids, I use a dilute solution of soapy water in an old hand sprayer that was once used for window washing.

Aphids seem to really slow down pepper seedlings. Another issue can be water-logged soil, which kills of a lot of the root structure. I'd let them dry out a bit, and then fertilize with any balanced organic fertilizer.

The local nursery here told me to plant 3 matches with the pepper plants - that they thrive on sulfur.......... Not sure how organic this is ? But its worth a try!

I could be wrong but I would spray the aphids off with water like posted above , every day or every other day. I would also use some bat guano for nitrogen. The nitrogen will boost growth and add other minerals but you have to keep a close eye on the aphids because like also posted already they thrive on plants that are given to much nitrogen. I would not use a spray solution for the aphids unless absolutely necessary and then a 1/4 the solution on the young plants. Add nitrogen to get your growth but take care of the aphids at the same time.

Thanks - that's the dilemma, these are very small and (even the tomatoes which also have aphids but are larger) light green, makes me think they need N but I want to get rid of the aphids b4 adding N. Don't have guano - how about weak coffee "tea" (made from used grounds and lukewarm water sitting in the sun)?