Keep in mind that peppers LOVE, LOVE, LOVE, LOVE hot weather. The hotter the better... If you put them out too soon, and the ground is still too chilly for them, they will sit, sulk, and maybe even, stall.
If the weather isn't getting close to being very summery hot now, I'd hold them inside awhile longer. It won't hurt them.
Also, your plants look a little pale-ish on a few leaves. Couple potential reasons: overwatering, or undernourishment. I know those pots dry out fairly quickly and overwatering can occur quite easily if you're trying to keep them hydrated. I would suggest setting the whole biodegradeable pot into a larger pot of mix, give 'em more room to grow on, and keep them close to the lights (you are using lights, yes?). Just peel off the bottoms so the roots can drop down into the new potting mix. They'll spread out. Won't hurt to transplant the seedlings out at 10-12" tall...
Are you feeding the plants? They have enough true leaves, now, to be asking for a meal or two, LOL! I'd start with a weak application of some water soluble fertilizer mixed in a gallon of water. Add it to the bottom of the drip tray and let the plants soak it up. Remove any excess after about 1-2 hours. Don't let the plants stand in water constantly. I use plain old Miracle Grow plant food. The blue stuff. I add enough to a gallon of water to barely color it, and feed the plants once a week. Feed weakly, weekly...
And, don't forget the lights! I use regular old fluorescent shop lights. You can prop them up on some books on each end. Keep them at least 1-2" from the tops of the leaves...16 hours a day...7am-11pm for my seedlings...
Don't prune peppers. The only reason to prune peppers is when they have gotten so big that they are interferring with other plants or have suffered broken branches from wind/cat races/grandchildren.
Be aware that they really are warm weather plants and will not start producing until late spring/early summer. In northern regions of south Texas (i.e. San Antonio/New Braunfels), they normally start setting fruit in May and are productive by June.
Bell peppers make the best peppers (IMO) when watered regularly. They don't want wet feet, but they do like to drink. When it gets too hot, they no longer produce well, but with a bit of luck you can make them productive again in early fall.
Jalapenos tolerate dry conditions, and the peppers are hotter when grown in moderately harsh conditions. So if you want very mild peppers, water with the bells. If you want something snappier, then let them go longer between waterings. For jalapenos, the hotter the temperature, the better they like it.
I started my bell peppers and cubanelle's about 2 months ago and have them outside in 5 gallon containers and they are doing great. This is South Florida's growing season and we've been having a warm winter so far. Two local places I like going for some great advice is marando farms and Urban farmer. Good luck!
Your peppers are ready to transplant now. If it is still cool outside, transplant them into larger containers and put them outside during the day. Don't prune at all, but fertilize with some "tea" made from well composted cow manure.
A good resource for more information:
The University of California at Santa Cruz has one of the most respected programs on sustainable agriculture in the USA. It's called the Agroecology Program, which all started with the work of Alan Chadwick back in 1967.
Alan had some very important teachings about the fine art of transplanting. One of these is to handle the seedling by one leaf only, never by the crown (the place where the stem meets the roots). That is one of the most fragile parts of the plant, and bruising it can seriously hamper the overall health of your seedling. The second point is never to get water on the leaves of your seedling for about three days after transplanting. Water on the leaves stimulates leaf growth, but what you want at this stage is for the roots to get well established.
More on Chadwick´s techniques and philosophy can be found here:
Soil temp for plant peppers out into the garden should be between 65 - 70 degrees with nighttime air temp not dipping below 55 degrees. A black plastic mulch material can help keep the soil temp warm and can be removed once the temperatures start to climb.