Ok I assume you have added the ship styles you want into FactionShipStyleDefs.xml and then created NEW entries in ShipClassDefs.xml simply using the ship designs and thumbnails from your set but otherwise using standard blueprints.
Whatever you do keep to stock blueprints if you are having crashes.
Let me point out the entries to change using some entries from Gauntlets files. Green are items to change, red are items that must remain the same or strictly reference stock names such as blueprints
In FactionShipStyleDefs.xml
<ShipStyleSet>
<InternalName>FulgarTradeholdsShipStyleSet</InternalName> **Change this**
<DisplayName>FTH Ship Style</DisplayName> **Change this**
<Icon>GC3_S_Basic_Ship_Icon.png</Icon>
<DisplaySampleObjGfxConfig>0-FTH Medium Survey 01_with omestupidnumber.objgfxcfg</DisplaySampleObjGfxConfig> **Change this**
<ShipStyle>Krynn</ShipStyle>
<StartingShipyard>BasicShipyard</StartingShipyard>
Other than those entries only change the <ShipClass> tags you want to alter.
Then in the ShipClassDefs.xml
<ShipClass>
<InternalName>FulgarTradeholdsScout</InternalName> **Change this**
<DisplayName>Scout</DisplayName> **Change this**
<Description>Scout</Description> **Change this**
<ThumbnailOverride>0-FTH Tiny Scout_EC98DB4C08A84738BBE15A2EFF55AFAA.PNG</ThumbnailOverride> **Change this**
<ShipHullType>Tiny</ShipHullType>
<ShipRule>Scout</ShipRule>
<ShipDesign>0-FTH Tiny Scout_EC98DB4C08A84738BBE15A2EFF55AFAA</ShipDesign> **Change this**
<AIOnly>false</AIOnly>
<AIShipClass>Scout</AIShipClass>
<StrategicIcon>Scout</StrategicIcon>
<BlueprintDef>ScoutBlueprint</BlueprintDef>
</ShipClass>
If the only things you change are the items I've pointed out then there should be no crashing unless you have a space somewhere or a missing bracket, so if it still crashes then start looking for syntax errors.
The biggest thing I can think of that would cause an outright CTD is if the <ShipClass>Yournamehere</ShipClass> in the FactionShipStyleDefs.xml didn't match an entry in the ShipClassDefs.xml