Yes, if it is just a heat lamp, it will make your seedlings grow very tall and lanky and weak. It's ok for germinating the seed, but once they have green leaves they need a certain spectrum of light that fluorescents supply but a heat lamp would not. Without enough light, they will not survive. It's much better to grow the seedlings a little on the cool side, (but don't let them get below @ 55) with lots of light than too warm in low light.
What are you planning to do with that many plants? By summer you'll be needing to pot each one up into successively bigger pots . . . know anyone with a nursery?
