Read multiple modbus devices information by multiple PLC
Modbus/TCP can be handled by a simple unmanaged Ethernet switch.  I have used this numerous times, although I reserve the "cheap" switch for bench testing and use an industrial quality switch (temperature rating, redundant power, status output) in the final product.
It's difficult to quantify risk as you have asked, since I have no knowledge of your application.  If it's for lab use, then I see no risk.  


Note that Modbus/TCP is not deterministic.  If timing (or guaranteed "on time") is important, Modbus/TCP might not be appropriate.  This comes into play when you are doing something like feedback control, where the Modbus/TCP is in the feedback and final control paths.  Again, not knowing your application I won't chase this thought further.

I have used gateways before and they work, although can be pricey.  Anybus X is one I've used but I'm not necessarily recommending it. 

A good alternative to a dedicated gateway is to use the same type CPU as in the PLCs and write a small program to handle this function.  It gives you more flexibility and the hardware probably costs less than the gateway.  I've done this on several projects and now will only use a dedicated gateway if my PLC doesn't support the other protocol.