Eras and settings are mostly to give battles a theme - if you just want to put a lance or two of 'Mechs together and fight it out, just because you like the way those machines look, there's absolutely nothing stopping you - even if your choices are as varied as putting a KV-2, Centurion Sho't, AMX-40, and an M3 Stuart in the same tank platoon. Like it's been said, it's not a WYSIWYG game, and as long as you have counters of some kind, record sheets, a map, and dice, you're buenolicious.
That said, a lot of folks like certain eras of the game for story reasons, and like to play out battles that might be in that era, from certain faction sides. Sometimes it's fun to limit yourself to what might be available to one side, and have an opponent who does the same, so that you can fight your heroic defense against those damn dirty Davion invaders. "How do I make this work with what I have" mindsets, basically.
But don't let any of that stop you from taking whatever you think is cool on the field, as long as your opponent is cool with it and as long as you're cool with what they bring. Have fun, don't worry, and don't roll snake-eyes.