What are springtails? Are they a good source of nutrients for Petey?
Springtails are small, hopping insects that make great food for small and young terrestrial newts and salamanders. They will probably be completely ignored by your large P.ruber.
What else can I give it to add a little variety to its diet?
Besides earthworms (which make a great staple diet), waxworms make for a great occasional treat.
Are they solitary, or can I keep a colony, or pair?
Caudates by nature are solitary, but multiple animals may be kept together, under the condition that a large enough tank is provided.