2 250w over a 55g

I think MH's are overkill on any freshwater planted tank unless you have a minimum of a 24 inch depth. If this was saltwater though, you'd be set.

I have 2 400w mhs from my salt water tank..you will get about 2 maybe 3 feet of coverage depending on the reflector. Dont get me wrong it will be intense light, but it wont cover an area any more then that.

If I were you I would get some t5's and save the money on electricity. However free is free, and if you dont have the money for t5's then go with the 250's. Just make sure you keep an eye on your temps, and you better start dosing and cranking up that co2!

Is there a way to dim the lights?

No. You can get window screening or egg crate (light diffuser) at HD/Lowes and use that. I use it to light acclimate my corals.

You're gonna be battling algae unless you start out HEAVILY planted with good CO2. I have 2x150W MH over my 40 gallon tank myself.

there is a psibility that I can get for a decent price a tek t5 8bulbs

I suggest going with the T5 lighting over the 2 x 250watt MH even though the MH are free.

Initially when i got my lighting for my 90g, I had 2 x 250watts and all my plants were growing horizontal over the substrate, none were growing upward due to the intensity and I had to replace them 2 weeks later for 175's instead.

2 x 250watts of MH is just too intense of a lighting for that size tank. You can diffuse the intensity with the methods epic suggests but it's not practical long term.