If you want to be really safe about it - use the exchange supported spreads and do not manually leg the spreads.
I'll just comment on this. Since some nice spreads are not exchange supported, you will have to leg them.
Examples
ES/NKD
NQ/YM
YM/RTY
NQ/ES
In general, if you want to trade index spreads or spread rate futures against indexes then you will be legging the spreads.
If you are leaving legs open for any length of time - minutes or even hours; then your FCM Risk Department is naturally going to margin those legs as outright.
This is one of the reasons that you will need a large account. This is also why it is generally considered a professional style of trading.