ok about first question the answer Yes you can
about the second question
the answer is really depends on ur network size, ur goal, the type off traffic and ISP link, and the budget as well
but let me give u an idea to know how to chose in general
from networking prespective it is better to have a router on the network edge because router can handel network traffic fron routing prespective better for example u can use policy based routing in a router while with the firewall u cant, i fu have more than on isp link u can do loadbalancing while with firewall u cant, also if u have routing with ur isp such as BGP firewall dosnt support BGP
as i told u u can use firewall but router much more flexable than a firewall
but also it depends if u have samll office with ADSL firewall enough in this case
furthurmore router on the network edge with VOIP better than firewall because router have much more support and features for voip and policies
FROM SECURITY prespective
it is better to have arouter in the edge to do packect inspection with ACLs this is first security layer
the second security layer is the firewall behind the edge router also known as permiter router
this firewall will inspect the permited traffic coming from the edge router
so this will make higher level of security
good luck
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