The iS145 is to wide for Slalom 42, so might be a problem unless your club/match racing.
Depending on your sail sizes, water conditions and type of racing (figure of 8 or downwind), you can be very competitive with two boards. iS122/iS94 (flat water/lighter winds) or iS111/iS87 (choppier water/higher winds). 3 fins per board will increase the already big range.
In general, the bigger combo is more suited to short leg/figure of 8 style racers as early planning ans acceleration is king. The smaller combo will give you better downwind speed and control, and also long distance as less physical to use.
Alot of the UKWA racers are using just 2 boards with great effect. Easier transport, lower initial outlay and more time on boards so you get to know the boards real well.
Just my penny's worth, Cheers
P.S. Hope you kick ass next race.