A 60 cm antenna is best but provided the smaller one is properly aligned it will work okay except in bad weather, where service should fail on this one before services on the bigger one fail.
You DO NOT need a diplexer for the Explora 1 IF it is connected to its own LNB. It is only required where it is also connected to the SLNB feeding the other two decoders AND you want to feed HB to the decoders via the LNB cabling.
I presume the older antenna has a TWIN LNB feeding an Explora switch which then feeds the Explora 1? If so, no changes to your install are required.
The HB cable to the older decoder is the only requirement.
Easyview, UEC 4T HD PVR, SD PVR, XV
Spare decoders: SD PVR(2), PACE HD PVR 4T, DSD 660, 1110, 1131, Explora
2 unmentionable FTA decoders
Win 10 Pro (64-bit) build 1607, version 14393.351
1.2m antenna, 8-way universal LNB, 2x6 MS, FSM permanently connected.