what weapon do you start with the light tank?

[Vomitus]

well then, what weapon do you start with for heli?

I usually go for graded basic magic. It is very weak in the beginning but you can quickly buy another one:).
And of course, heli's maneuverability is nothing without long range. As my experience shows even 950 range is not enough(for me:)), 1000 - is best choice.

Btw, as far as requirements for high rank tanks were added in 8.36 extra long range weapons with air units became more valuable. When in 8.35 enemy could quickly buy inf and you can nothing to do to him with your long range now you can easily kill low-hp tanks with your quite expansive long-range and don't to get worry about your 2k-3k hp:)