When you get into a fly you buy and sell an equal number of spreads. Going long a 6 month spread is the sum of going long six 1 month spreads, you could build it that way by buying each month individually but then you'd be paying more commissions. So in order to hedge that you need to equivalently sell 6 lots of a 1 month, that's all it is, you just need your exposure to be equal.
Also I only talked about momentum in reference to legging into a trade to improve execution edge.
mmt
Based on a 6 lot trade. Getting into a spread will cost 6 roundtrips. Getting into a 1 month fly will cost 6 roundtrips to get into 1 leg and 6 for the other leg. Getting into a 1v6 combo will cost 6 roundtrips for the 1month and 1 roundtrip for the 6 month - so an extra roundtrip, or 1/6th more expensive than a straight spread trade.