Need An Advice

We have an Juniper MX80 router  , one SRX 3600 and one Ex4500.

What if you were me, How do you design your network and where do you locate the Vlans ?

* do you set gateways on firewall
* do you set vlans on MX or EX
* do you transfer traffic with BGP / Static route
* do you use metric connection ?
....

I just really need an advice.

Thank you