Micro Seiki ma-505 x is a low or heavy tonarm?

You are probably right.... to me, 7 gr seems a bit  too light to be the full effective mass

 Here is a discussion about that arm. The head shell weighs 9.7 grams!    https://www.vinylengine.com/turntable_forum/viewtopic.php?t=48848

I own a Micro Seiki DD 40 with dual tone arms Both MA505. One has the Ortofon Black and the other has the Denon DL 103R. My question is as long as you are able to balance the tonearm is the tonearm mass a non factor. Thanks

No. Tonearm effective mass and cartridge compliance determine the resonant frequency of the ensemble. Has nothing to do with balancing except insofar as the counterweight and its distance from the pivot do affect the effective mass.