[quote]1) Is there option in PL-SQL developer to connect to db without having TNS entry, like the one we have in TOAD.[/quote]
You can use the EZCONNECT naming method (host[:port][/service_name]) if Oracle Net is configured as such, or you can enter an entire connect descriptor (just like in a tnsnames.ora file) in the Database field.

[quote]2) Can we connect to Oracle XE through PL-SQL developer?[/quote]
Yes, no problem.