Adding a fourth string, looking for options.
I have 3 identical solar panels on the roof of a 5th wheel RV (340W, 41.2 Vmp, 8.26 Imp) in parallel (running on individual 10G wires to a combiner box that has 10A fuzes, additionally each positive leg has a MC4 15A fuze at the panel connections themselves), I want to add an additional solar string on an extension at the ground (max 50 ft, current strings are 15, 20, and 30 ft long), for extra power if needed and in case of shading of the roof. I am using a Victron 150/100 MPPT Tr, so it should be able to handle the 4th string. I want to use 2 or 3 panels in series (so they are smaller and easier to store when not used) to equal the others, then connected in parallel with them. I know that they need to add up to (within 10% of) the same voltage as the current panels, but everything will be producing at the lowest values (V in parallel or I in series) when connected.
Are there any specific things that are more important to pay attention to (or anything else I'm missing) in my plan? Is there a problem if the 4th string is poly panels instead of the mono I already have? Any recommendations for cheap panels to do this? I'm in AZ so local is preferred and I originally purchased my panels from the forums sponsor.