When developing my User Interface on Android or Web BrowserNX, is there any UI guidelines that I need to follow?

Yes, in the Resource CD there are two guidelines that highlight information on the best practices with regards to developing your User Interface on the Ricoh SmartSDK.

Resource CD reference:  Common Information > Overview > Design Guidelines > 
UI Design Guidelines

URL location: ([root location of CD]/doc/en/common/142-01-0200.htm?ssdk_v2)