Do I need to see the original to understand Plus?

Nah, none of the characters appear, and the resolution to the original show as well as what exactly the Macross is expalined well enough.
It's more of a personal tale between two test pilot rivals, so the backstory of the franchise is largely irrelevant.

I enjoy Macross Frontier and 7, but that's just me. Macross 7 is the first Macross show that I seen (not counting Robotech).

7 is episodic as heck, but it's hard not to love it if you are into the premise.
Frontier however, I just love it, can't even be objective about it.

a good mecha series you could check out, if you haven't seen it, is Big-O.

Big-O is a great series. gotta love big romantic metal piston fist mechs.
http://stream1.gifsoup.com/view1/341...o-piston-o.gif

I like Big O, but I never like how it ended though.

Yeah, the ending was pure ****. It also doesn't help that Sunrise left it open for a third season in hopes that CN would order one instead of just completing the ****ing story like they were originally going to.

they wanted the American audience to enjoy it and they succeeded. they said they didn't care if the japanese audience didn't like it.

and I like the ending.

Big-O reminded me of a cross between Batman/Bruce Wayne and Gundam.