I keep my (tender) autumn and winter sown plantlets in my cold greenhouse until May. I like to maintain control over irrigation and pest protection....I don't think lack of heat is much of an issue but if cold nights threaten, I drop a bit of horticultural fleece over thunbergias, morning glories and other semi-tender plantings.

It's still very early, so don't give up hope on the 2 that haven't germinated. The perennials should be ok outside if the temperature doesn't fall below freezing, and tropical plants shouldn't be left outside until the night temperstures remain above 45. Sweetpeas can handle 22 degrees F. Were the milk jugs indoors? You said you were putting them outside, but I consider winter sowing to mean the jugs will be outside the whole winter? If they've been germinated inside, than you should harden them off for a few days first. You will be removing the jug tops when you put them outside correct?