multi microcontrolers and one ram for use like a circular buffer for intercomunication, how do and select?
There are many issues here. The first is that many of these modules communicate with serial interfaces already, so if you want to dedicate a memory interface to each device, you'll have to add some logic (maybe even extra microcontrollers) to translate. Then, most RAMs are single ported, so you can't connect them directly to two devices. Either you will need an arbiter or you will need to get very expensive (and rather small) dual port RAM.

Or you can just use the hardware serial ports on your microcontroller and implement your circular buffers with software in the chip's on board RAM.

Now, if your chip doesn't have enough hardware serial ports for all the devices that you want to connect, then it might be worthwhile to add a secondary peripheral microcontroller that has more serial ports and can perform operations on behalf of the main controller and then aggregate responses.  Something like the Atmel xmega series could be a good choice as these chips have gobs and gobs of serial ports (up to 8 UARTs, multiple I2C and SPI controllers, decent amount of RAM, will run at 32 MHz on the internal oscillator, etc.).  Then all you need is a relatively fast interface between the two microcontrollers.  A fast UART or I2C bus would be reasonable.  Or you could even use USB if your main controller can act as a USB host.