Lets go over the different setups and what they can do for you:
Multi mains and ploy crosses:
Multifilaments are designed for comfort above all else, with a secondary benefit of having slightly better tension maintenance than syngut. Poly is the compromise between the stiffness of Kevlar and the elasticity of nylon.
Having multifilament mains in a hybrid is counterproductive because you're not gaining the spin-potential of the stiffer poly, and any increase in the overall life of the stringbed is marginal, at best. All you're doing is lowering the power of the stringbed.
Poly mains and multi crosses:
This setup has greater potential with the poly in the place where you would need it. The mains. Multi crosses soften up the feel somewhat, though your stringer is correct in saying the mains influence the feel of the stringbed more. The real benefit of multi crosses here is the nylon material is more flexible, thus allowing the stiff mains to bend out of place and snap back more easily.
A few words about these two particular hybrid combos vs others:
There are really only a couple hybrid setups that hold signifigant advantage over a full bed of whatever...
Gut mains and poly crosses:
Gut has the best resiliency (elasticity) of any string material. It stretches better and does so longer and more consistently than nylon, Kevlar, poly, piano wire, you name it. It of course is also the most comfortable. Gut is an ideal material for mainstrings when combined with poly...
Poly crosses are VERY slippery and pairings like this work extremely efficiently because the inter-string friction is so low. Poly crosses lower the power by stiffening up the stringbed (how much the string bed will deflect upon impact), but increase control and allow gut mains to really stretch and snap back like nobody's buisness. This setup actually gets better as you play because as the poly loses tension, those mains have an even easier time moving in and out of place. It is recommended to use string savers to prevent the poly from sawing through the mains.
Poly mains and gut crosses/syngut crosses:
Poly is stiff and snaps back into place with great energy after impact. A soft, flexible cross allows the mains to do this more easily. Not quite as efficiently as low inter-string friction (which this setup still has, with a few drawbacks), but it nevertheless helps. The difference between this and the reverse setup is that poly loses its elastic qualities FAST. The harder you play, the faster it goes dead. Having a soft cross here will make this more noticeable because the dead mains will stretch, and not snap back at all because there's nothing forcing it to do so. You will no doubt read on this forum how many poly users above 4.0 will restring after as little as 8 hours. Pros change their Racquet every half hour. Thus doing what they do is not economically feasible unless you're touring for money.
Kevlar mains and non-kevlar crosses. Extremely stiff and unforgiving but booooy, the spin is fantastic. Generally not worth the level of shock transfer to your arm.
Syngut mains and poly crosses:
Another hybrid that's not worth experimenting with. Syngut doesn't have the elasticity of gut, nor the stiffness of poly. You gain nothing here over a full bed of nylon.
PS85, 367g/345sw, Pacific classic@58