I have some connection servers 7.1 with 2012R2,
1) Can I install new replication server 7.5.2 to join connection server 7.1?
2) Or Can I export connection server 7.1 config to version 7.5.2?
Do you want to do this as part of an upgrade path for your environment?
Here is the Horizon Upgrade overview KB
Depending on your needs you could install one more 7.1 connection server then upgrade it to 7.5.2 so that it is still part of your Pod. Then after testing make a plan of action to upgrade the remaining servers to match.
VMware does not recommend staying on different builds of Horizon for long but it supported as part of an upgrade path.