For your question, I personally don't think there should be a big problem. The suggested ratio of Ca++ and Mg++ intake is 2:1; if your Mg++ intake is times more than Ca++ intake, it shouldn't be a problem for Mg++ intake. OK, even it might decrease Mg++ intake a bit, you can always add a Mg tablet for that.
To be honest, I don't trust ZMA that much. It might be really good to take enough Mg++, Zinc, B6, but I can't see why it has to be that ratio. Moreover, I can't see why it has to be that kind of compound: if you search on google, you will find the bio-usage ratio of common used MgO2; yes, it's lower, about 1:1.6, but you can take 0.6 times more to adjust that, and the price is more than five times different.
You can get a bottle of dirt cheap MgO2 tablet, a bottle of dirt cheap Zinc Glucoside tablet, a bottle of dirt cheap B6, which in a sum cost 1/3 of your ZMA....
There are also many papers (published by independent groups rather than manufactur sponsed ones) doubt the effect of ZMA's increasing Testo level.
Does anyone who takes just ZMA and found testo level is boosted? (surely that ZMA product has to be free from any illegal ingredient)
For me, that combination makes me sleep better, but Mg++ only can do that too; and I got slightly better 'morning wood', but B6 only can do that too.