Hi Joshua,
"connect the 10Gps lines to the corresponding 4 RX and 4TX lines meant for the 28Gps lines for the 100G."
Do you means that the 4x10G MAC IP connect to the QSFP+ and then connect to QSFP28? if yes, you only will get an output of 40G since the maximum 10G will be given out from MAC.
For the multiple 10G MAC with multirate PHY, I utilized the example design that is generated from the LL 10G MAC IP GUI and then add 2 more 10G MAC+PHY into the example design. It was being compiled successfully from my side.
If you wish to implement 100G design, your design should be 25Gx4. I would like to suggest you to use either 25G IP or LL 100G IP.
Best regards,
Zi Ying