I formed a team of folks last year that tested and demonstrate CLogix and S7 comms three different ways to see which way worked best. Two of the ways untilized Ethernet and one way PROFIBUS.
Ethernet
1) Using a CLogix with a EWEB module we configurated (with much help from RA and a customer) native socket comms with a S7-315 PN/DP and passed a few hundred bytes of data between the two. It took sometime to make this work. Pros: it worked and worked rather well Cons: EWEB module cost, ENBT had to be used in addition to EWEB because of Ethernet/IP IO was needed too, and the biggest draw back was the amount of code, configuration and complexity of the comms between the two.
2) Using a PROFINET proxy from Woodhead
http://213.130.34.122/pdf/en/applicom_PROFINETIO%20Proxy.pdf we setup a configuration for both Ethernet/IP and PROFINET and passed a few hundred bytes between the two. Pros: easy to setup, cost was better than expected, performance was pretty good. Cons: another device, another configuration, another point of possible failure
PROFIBUS
3) Using a Woodhead PROFIBUS module
http://213.130.34.122/pdf/en/SST_PFB-CLX-RLL_FDT.pdf in the CLogix rack we setup a few hundred bytes of data between the CLogix and S7 using PROFIBUS. Pros: best performance, easy configuration, lowest cost option, clean layout. Cons: another network link, but our application allowed for only a meter of PROFIBUS cable to be used between the two controllers so it was not that big of a deal.
The final decision was made to use the PROFIBUS configuration because it was the lowest cost, best performance, easiest to setup and maintain.
Key points, the native socket comms on the RA side required EWEB module which didn't support EIP IO traffic so another ENBT was needed too. Both the S7300 and 400 family of PROFINET CPUs supported native socket comms and all of the CP343 and CP443 support native socket comms. If using the Woodhead PROFINET proxy required that you used a PROFINET enabled CPU or a CP343 / CP443 PROFINET supported CPU.
Another option if you rather have a software solution. Use a PC with OPC DX servers for both CLogix (EIP), and S7 (Ethernet S7 Communications, or PROFINET). OPC DX (Data Exchange) allows bridging between two OPC server so the PC becomes your traffic cop (bridge). I've not done or seen this done with CLogix / S7 comms, but have seen this done with TI / S7, and Modicon MB+ / S7 and it worked rather well.
For your application with (2) CLogix and (1) S7 I would look at either the proxy or PROFIBUS in the CLogix racks. It took a great deal of code and hand shaking between (1) CLogix and (1) S7 in my test to make this work.
Good luck on your path forward.