some techmarines may be in the crew of space marine vessels aside from serfs and servitors
the navy normally uses frigates to accompany cruisers
space marines have fewer options
strike cruisers/battle barges are normally geared towards planetary assaults
vanguard strike cruisers for ship to ship combat and would be an option for patrols, for which they also might be used
, however
most space marine chapters use Nova Class Frigates for patrols/scouting missions, as void combat between starships is not really their main role anymore but taken over by the imperial navy.
also a cruiser will rarely be left without escorts as otherwise it would be very vulnerable to certain attack forms
also, think of it this way... chapters do have their areas of space and access to manufactorums that will be used solely for their purposes (see f.e. astral claws and maelstrom zone), as such they might possess their own shipyards (f.e.e the ultramarines do)